Resins and rubbers (hereinafter referred to as resin, etc.) are light in weight and can be molded easily at a relatively low temperature compared with other materials but, on the other hand, they involve such drawbacks that they are deteriorated remarkably by UV-light and are poor in mechanical strength and gas barrier property. On the other hand, while glass is excellent in chemical stability and has high mechanical strength or gas barrier property compared with the resin and the rubber but it has, a drawback of tending to cause brittle fracture and being heavy in weight.
Accordingly, composite materials comprising resins and glasses to compensate their respective drawbacks have been invented as typically represented by glass fiber reinforced plastics (GFRP). However, since adhesion between an oxide such as the glass and the resin is low, the strength at the boundary between them predominates the mechanical strength of the composite material.
For example, a patent literature 1 discloses that a sizing agent containing a silane coupling agent and a resin is coated on the surface of glass fibers.